1. Field of the Invention
The present invention relates to a semiconductor laser driving circuit which is used in an optical disk recording and reproducing system wherein information is recorded on and reproduced from an optical disk.
2. Description of the Related Art
In the conventional optical disk system for recording or reproducing information with a laser beam, in a recording mode, it serves to follow the steps of intensifying or mitigating a laser output pulsewise in accordance with information to be recorded, changing the power of a laser beam spot restricted on a disk by an optical system accordingly, and changing an optical or magnetic state of a beam-irradiated spot of a recording film formed on the disk for recording the information. In a reproducing mode, it serves to follow the steps of irradiating the disk with a laser beam having such an intensity as not transforming the recording film and detecting the reflected beam for reproducing the information recorded thereon.
It will be understood from this function that it is necessary to keep a laser output at a predetermined value in both the recording and the reproducing modes. Because the semiconductor laser is subject to the change of an ambient temperature of the used semiconductors, the laser output is required to be controlled so that it can avoid any influence affected by the temperature change or the other factors.
FIG. 1 is a circuit diagram showing a conventional laser driving circuit, which mainly includes a control logic circuit 11, a reproduction power producing circuit 12, and a recording power producing circuit 13. Further, it is seen from this figure that a laser 3 and a photodetector 4 are also added to the circuit. The photodetector 4 may employ a photodiode. As shown, the control logic circuit 11 is connected to a +5 V power source and the reproduction power producing circuit 12 and the recording power producing circuit 13 are connected to a +12 V power source.
The reproduction power producing circuit 12 includes a transistor 2 for driving the laser 3 in a reproducing mode, a current-to-voltage converting circuit 5 for converting the current generated in the photodetector 4 into a voltage, an operational amplifier 6 for amplifying the converted voltage, and a switching circuit 1 for switching on and off a contact between the operational amplifier 6 and the transistor 2.
The recording power producing circuit 13 consists of a transistor 9 for driving the laser 3 in a recording mode, a recording power setting circuit 8 connected to the base of transistor 9, and a switching 10 for switching on and off a contact between the transistor 9 and the laser 3.
In operation, in the reproducing mode, the control logic circuit 11 supplies the reproduction signal a to the switching circuit 1, thereby switching on the switching circuit 1 and activating the laser 3. Then, the laser 3 radiates a beam c to the photodetector 4, which generates a current d. Then, the current d is supplied to the current-to-voltage converting circuit 5 in which the current d is converted into a voltage e and is amplified by the operational amplifier 6. The amplified voltage is supplied to the transistor 2 through the switching circuit 1. The transistor 2 serves to adjustably produce the reproduction driving current I.sub.R. In summary, this operation can be summarized as detecting the laser beam and applying negative feedback to the detected signal for keeping a driving current constant.
In the recording mode, on the other hand, the recording power setting circuit 8 supplies a predetermined signal to the transistor 9 in which the recording driving current is adjtably produced, while the control logic circuit 11 supplies the recording signal b to the switching circuit 10. The switching circuit 10 is switched on and off in accordance with the magnitude of the signal b. The on-and-off switching results in modulating a recording driving current I.sub.W and superposing the recording signal on the driving current I.sub.W. At this time, for interrupting the control operation, the reproduction driving current I.sub.R is kept at the value given immediately before executing the operation in the recording mode by a sample holding circuit (not shown).
As mentioned above, the foregoing semiconductor laser driving circuit has two or more power voltages. That is, the control logic circuit 11 is connected to the +5 power source and the reproduction power producing circuit 12 and the recording power producing circuit 13 are connected to the +12 power source. In this case, the switching of the power sources may result in bringing about a time lag between the leading and the trailing edges of the former power source and those of the latter one.
FIG. 2 is a graph showing the state of a voltage caused when the time lag appears, in particular, when the power sources are interrupted in driving the laser. In the reproduction mode, assuming that the power sources are interrupted at a time t.sub.0, as shown in the curves A and B of the graph, the +5 v power source connected to the control logic circuit 11 supplies the lowest possible operating voltage for activating the logic circuit 11 at a time t.sub.1 and the +12 v power source connected to the reproduction power producing circuits 12 and 13 also supplies the lowest possible operating voltage for activating the circuits 12 and 13 at a time t.sub.2. At an interval of .DELTA.t between the times t.sub.1 and t.sub.2, the control logic circuit 11 cannot keep its logic level stable, thereby switching on the switching circuit 10 as shown in the curve C and supplying the recording driving current I.sub.W as shown in the curve D to the laser 3. It results in bringing about a drawback that the laser output reaches such a power that it can transform the recording film formed on the disk and thus gives damage to the information recorded on the disk.